Conventionally, in electric motorcars, a drive unit that is used to transmit the power of a motor to drive wheels to drive a vehicle generally comprises an input shaft, a countershaft and an output shaft. The input shaft is coupled with the output shaft of the motor, which provide a driving power, and the countershaft is rotationally connected to the input shaft to transmit the power from the input shaft. The output shaft of the drive unit is rotationally connected to transmit the power from the countershaft to the drive wheels. For the rotational connection, gears are provided around the respective shafts in the drive unit, so that the power of the motor is transmitted to the drive wheels. In addition, bearings are provided to rotationally support the respective shafts at both the ends thereof inside the drive unit (refer to, for example, Japanese Laid-Open Patent Publication No. H08(1996)-230489).
Among those bearings, a second bearing that supports the motor-side end of the input shaft of the drive unit functions also as a supporting point for the output shaft of the motor, so the inner ring of this bearing is fit over the output shaft of the motor. The output shaft of the motor and the input shaft of the drive unit are fit by splines to securely transmit the power (torque). Spline fitting is a coupling method in which multiple tongues and grooves like keys are provided axially at even intervals around respective shafts that are to be coupled, and when these shafts are coupled, the fitting allows each shaft rotating to move axially within a certain range. Excluding serration fittings, which have no clearance and are intended for permanent fit, spline fittings are generally looser and the degrees of their concentricity are lower than ordinary fittings of shafts and bores.
Therefore, when a high-frequency vibration generated at the motor is transmitted from the output shaft of the motor to the second bearing, it is difficult to restrain the vibration there. Moreover, the vibration is often intensified there, and it propagates to the motor case. As a result, the motor case happens to vibrate like a membrane as if it were a speaker.
One method to prevent such a problem is to press and fit the splined parts as practiced in the fitting of the countershaft, so that the rotating shafts may be avoided of vibrations. However, the application of press-fitting for the input shaft reduces the workability for mounting the drive unit onto the motor case. Therefore, it is difficult to adopt this method immediately at this point.
Another idea is that, instead of the single second bearing, two bearings may-be placed to support the output shaft of the motor and the input shaft of the drive unit separately, and a spline fitting may be provided to couple these shafts between these two bearings. The placement of the two bearings and the spline fitting require a relatively large space, so the size as well as the weight of the drive unit must increase. Therefore, this method is not preferable.
As a method for reducing sounds coming from the drives of electric motorcars, a vibration-reducing technology for a vehicle-driving motor as prime mover (hereinafter referred to as “electrical motor”) is known for vibration and sound insulation and absorption (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-88035). By the way, it is typical in electrical motorcars that an electrical motor is combined with a reducer, and they are mounted as a unit in a housing. In this design, the vibration of the electrical motor propagates to the housing, where the vibration can be intensified and emitted as a loud sound. The transmission path for the vibration from the electrical motor is analyzed as follows. A magnetic vibration (magnetic strain) is generated at the stator coil of the electrical motor, and this vibration is transmitted to the housing. Also, a magnetic vibration is generated at the rotor magnet and is transmitted through the rotor shaft to the housing. The vibrations at the housing are emitted as sounds (hereinafter referred to as “radiating sound”) especially from the relatively thin part and from the part which is inferior in stiffness because of their structural simplicity.
To prevent radiating sounds, which can intensify at the housing and are emitted therefrom, there is a method for vibration insulation that cuts off the motor sound by providing the housing with an insulator and by increasing the stiffness and thickness of the housing. Another method for vibration insulation is to provide the housing with a weight, which is used for adjusting the resonance point of the housing, so that the resonance range of the housing is shifted to avoid vibrations within a range of practical rotational speed of the electrical motor.
However, all the above mentioned methods have a problem of increasing the size and weight of the housing.